The invention relates to an artificial finger joint comprising a convex joint head and comprising a concave joint shell which can be fastened independently of one another with a respective shaft in a bone end and which can be moved in an articulation plane from an extension position with parallel shaft axes into a hyperextension position or into an articulation end position.
Artificial finger joints occur between the metacarpal bone and the finger bone or between individual finger bones. In order to be able to operate at a finger joint of this kind, the surgeon must expose the bone ends and push aside minute blood vessels, nerves and tendons without overstretching them. It is therefore usual with the lateral pushing aside, which would cause a stretching, to bring the joint into articulation position, in which both bone ends are exposed for a resection. Accordingly it is an advantage when the joint head and the joint pan can be inserted independently of one another and can be brought into engagement as in a natural joint without the above mentioned overstretching arising.
Thus the patent specification U.S. Pat. No. 4,231,121 shows a finger joint which consists of a joint head with a formed on shaft and of a joint shell with a formed on shaft, which can be pushed together in an articulation position. The freedom of movement goes from an extension position into a hyperextension position or into an articulation end position. An embodiment of this kind has the disadvantage that in the end positions, for example in the hyperextension, a sliding off of the joint shell from the joint head can take place when a transverse force arises since the equilibrium state depends substantially on the force with which the two joint parts are held together by the ligaments which surround them.
The object of the invention is to improve artificial finger joints in this regard. This object is satisfied in that a guide pin projects out of the joint shell in the direction of its shaft axis and protrudes into a pocket of the joint head, with the pocket having a first abutment for the guide pin in the hyperextension position; and in that a second abutment is present between the joint shell and the joint head in the hyperextension position and prevents a tilting of the guide pin and the shaft of the joint shell about the first abutment.
The invention has the advantage that, through the provision of a second abutment in a position in which a first abutment is reached for the hyperextension position and would provide a momentary center for a continuation of the rotation, the movement is stopped.
Further advantageous improvements result from the subordinate claims 2 to 12.
Thus it is advantageous that through a corresponding design of the pocket, pivotal angles xcex1 from 80xc2x0 to 130xc2x0 between the hyperextension position and the articulation end position are possible which lie in the pivotal range of a natural finger joint.
One embodiment provides a cap which is formed on at the joint shell in the direction of the hyperextension position and which when the hyperextension position is reached encounters a second abutment at the joint head which is displaced by a distance S beyond a point of rotation at the joint head in the direction towards the shaft of the latter. An advantage of this cap is that it holds specific ligaments at a distance and is at the same time guided by these ligaments. The guide pin can project from the joint shell as a round pin, which is simple in the manufacturing technology. A further advantage of this cap consists in that the palmar luxation is suppressed in a grasping movement.
A further embodiment provides for the pocket having beyond a point of rotation an undercut extension with a second abutment which is arranged oppositely to the first abutment in order to stop the guide pin, which is extended beyond the point of rotation, when it encounters the first abutment. This embodiment has the advantage that the second abutment lies within the joint head and can not disturb externally.
Between the guide pin and the pocket, a clearance from the lateral guiding cheeks can be provided which amounts to more than 5% of the width of the guide pin in order to enable an articulation in a laterally slightly angled off position of the shaft axes. This has the advantage that fingers can articulate parallel to one another in spite of the splay position of the metacarpal bone.
With a rectangular cross-section of the guide pin the clearance from the guiding cheeks can be adjusted in such a manner that a rotational securing is achieved through a rectangular diagonal which is longer than the spacing of the guiding cheeks. A guide pin which is reduced by the amount of the clearance from the guiding cheeks has the advantage that the degrees of freedom as in a natural finger joint are present for the joint movements.
A further advantage consists in that the bearing surfaces of the joint head and the bearing shell are congruent in the hyperextension position in order to be able to take up the greatest bearing forces in this position, that the curvature of the joint head may however increase in the articulation direction in order not to tension the ligaments too strongly during the articulation.
Analogously to the restriction of the movement in the hyperextension, a third abutment can be provided in the articulation end position and a tilting about this third abutment can be prevented through a fourth abutment, which for example is likewise provided in the pocket and acts on the guide pin.
In order to realize a pocket with undercut abutments in a simple manufacturing technology, a pocket without undercuttings can first be produced in which subsequently provided bolts form the undercuttings and the abutments. The joint head and the joint shell are advantageously manufactured of body compatible metals, for example of titanium or titanium alloys or cobalt-chromium-molybdenum alloys. The frictional relationships between the articulation surfaces can be improved when one of the surfaces consists of plastic, for example of polyethylene or PEAK (polyaryl ether ketone).